1. Field of the Invention
The present invention relates to control systems and methods for hybrid electric vehicles.
2. Description of the Related Art
In a hybrid electric vehicle, a plurality of torque sources are available. Typically, such sources include an engine such as an internal combustion engine, and an electric machine. In one general topology of hybrid electric vehicles, the electric machine is a motor/generator interposed between the engine and the vehicle's transmission. The motor/generator can add torque to supplement the torque provided by engine. Further, the motor/generator can act as a generator in order to convert excess engine torque into electric energy for storage in a storage device such as a battery. This function as a generator can be in furtherance of the driver's request for the vehicle to decelerate (i.e., regenerative braking). The motor/generator can also act as a generator in a manner transparent to the driver of the vehicle, in order to assure that the battery maintains a reasonable state of charge.
Coordinated control of the engine, motor/generator and transmission is, of course, paramount for excellent vehicle performance. In one possible control partitioning, three controllers can be provided: an electronic engine controller (EEC), a transmission controller (TCM) and a vehicle system controller (VSC). In such a partitioning, the EEC would provide generally traditional engine control functions. The TCM would also provide generally traditional transmission control functions. The VSC would take accelerator position, vehicle speed, battery state of charge and other variables into consideration and partition a driver-commanded torque (as expressed primarily by accelerator position) into a desired motor/generator torque and a desired engine torque.
The driver-commanded torque would be provided from the VSC to both the EEC and the TCM. Because the TCM needs actual torque at its input for its control as well, the VSC provides a signal reflecting the sum of actual motor/generator torque (which the VSC knows because it performs control of the motor/generator) and actual measured engine torque or estimated engine torque (estimated within the VSC). When a transmission shift is impending or underway, the TCM provides a commanded transmission input torque to the VSC, which acts to cause the torque at the input to the transmission to conform to the command. This commanded torque allows the TCM to perform its shift as appropriate.
There are several concerns with the aforementioned control method. First, it requires the VSC to be able to anticipate dynamic effects, such as, for example, the effects of dynamic fueling strategy and manifold air flow, on actual engine torque, whereas in fact the EEC is the controller having the best knowledge of that information. This may be addressed in two ways, neither entirely satisfactory: 1) an engine torque sensor may be included in the system, or 2) the engine torque may be estimated within the EEC which has far better access to all engine control, sensor and calibration variables, as well as the control strategy itself. The first is an unusual and expensive solution, while the second will introduce a time delay during which the engine torque is computed in the EEC, passed to the VSC for combination with the starter/alternator torque and then passed to the TCM. This time delay will result in poor transmission shift quality. These specific issues derive from the genesis of this hierarchical control in a parallel HEV where the engine and electric machine each contribute substantial torque to propel the vehicle either separately or in combination. In such a case, it is absolutely essential that the VSC intervene completely in the connection between driver demand and the actual engine control inputs. In another type of HEV, the so-called "low storage requirement" or LSR hybrid having the topology described above, the torque contribution of the motor/generator is very small compared to that of the engine, and the vehicle has no electric-only propulsion mode at all. Thus the LSR system can take great advantage of the familiarity and proven base of control methodology of the more accustomed direct driver control of the engine with less severe intervention by the VSC. Such a strategy also allows use of non-hybrid powertrain components and controls with minimal modification.